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Corrosion protection of copper surfaces by an atmospheric pressure plasma jet treatment
Surface & Coatings Technology 205 (2011) S355–S358
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Corrosion protection of copper surfaces by an atmospheric pressure plasma jet treatment Christoph Regula ⁎, Joerg Ihde, Uwe Lommatzsch, Ralph Wilken Fraunhofer Institute for Manufacturing Technology and Advanced Materials, Wiener Str. 12, 28359 Bremen, Germany
a r t i c l e
i n f o
Available online 3 April 2011 Keywords: Atmospheric pressure plasma Copper Corrosion protection Silicon-organic plasma polymer coating
a b s t r a c t The pre-treatment of copper by an atmospheric pressure plasma jet system with different gas mixtures was studied. Their influence on the protection properties of plasma polymer coatings against corrosive environments were examined via an electrochemical test for the visualization of defects in the plasma polymer coating. The defect density could be significantly reduced by the pre-treatment with nitrogenhydrogen plasma. Time between pre-treatment and coating process is of important influence on the defect density as our investigation shows. The coating properties were compared with a typical protective paint system used today for electric boards. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Electronic devices consist of metallic conducting paths and contacts, mostly made from copper (Cu) due to its low electric resistance. These metallic parts have to be protected against deterioration by humidity or corrosive environments, therefore protective paints, e.g. parylene or polyaniline based coatings with thicknesses up to 100 μm are used [1–3]. Disadvantages of these processes are the consumption of solvents, the time necessary for curing and the required amount of energy. Due to the high thickness, protective paints are detrimental for the heat dissipation of the electronic components. Besides a lacquer coating a complete cover consisting of potting or mold compounds is used [4]. In some cases metallic surfaces can be protected by deposition of different metallicoxide layers e.g. [5,6]. Therefore we studied the use of an atmospheric pressure plasma (APP) jet system, operating in the non-equilibrium regime, as an alternative process for the deposition of protective coatings on technical Cu surfaces covered by oxides. The protective properties of the plasma polymer coating on Cu surfaces were studied by the analysis of inhomogeneities and electrochemically active defects in the coating. The generation of thin plasma polymer coatings by our APP jet is an environmentally friendly process, which does not rely on rare gases. The use of such APP jet systems for coating processes is an active research field [7–21]. Former investigations for corrosion protection of aluminium alloys with the same plasma device have been reported in [22].
For the experiments a modified version of an APP jet system (Plasmatreat GmbH, Steinhagen) was used (Fig. 1). This system operates with an excitation frequency between 17 and 25 kHz and a working gas flow rate of 1740 Nl·h− 1. Pre-treatment and coating is performed by moving the nozzle relative to the substrate at a speed between 10 and 20 m/min, which result in typical interaction times in the millisecond range. The samples were pre-treated applying compressed air (CA) or gas mixtures of nitrogen with hydrogen (N2/H2). Subsequently the samples were coated via plasma polymerisation. For all process steps the distance between the nozzle and the substrate was kept constant. For the deposition of the plasma polymer coatings, hexamethyldisiloxane (HMDSO, Wacker, purity N99%) was evaporated and injected into the plasma at the end of the nozzle by using nitrogen as process gas with a flow rate of 60 to 180 Nl·h− 1. The flow rate of the HMDSO was varied between 10 and 50 g/h. Flow rates of precursor and gases were controlled by mass flow controllers (Liqui-Flow (L23V02) and El-Flow (F201CV), Bronkhorst). The typical coating thickness was approximately 450 nm and showed a composition of ~ 25 at.% C, ~26 at.% Si and ~ 49 at.% O, by XPS-measurements. Scanning electron microscopy (SEM, Leo 1530 with a Gemini column, Zeiss) and X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos) of the surface were carried out to detect changes in surface topography and chemical composition. For corrosion tests, samples with 600 nm thick PVD Cu films on silicon wafers b100N (Si-Mat) were used. To study the protective properties of the plasma polymer coating against corrosive media a silver nitrate (AgNO3) solution of defined concentration and volume was applied. The silver ions undergo redox reaction with the Cu, e.g. at defects or conductive paths in the coating, leading to the deposition of silver (Ag) particles.
⁎ Corresponding author. Tel.: + 49 421 2246 682; fax: + 49 421 2246 666. E-mail address: [email protected] (C. Regula). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.126
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C. Regula et al. / Surface & Coatings Technology 205 (2011) S355–S358
Fig. 1. Scheme of the APP nozzle system for pre-treatment and plasma polymer deposition.
The localised depositions of Ag at the defects were detected by light microscopy and have been quantified by an optical automated analysis (Analysis, Olympus). Test duration of one hour is sufficient due to the high electro-chemical dynamic. After that time the particles continue to grow in size but no additional particles will be generated. This test method allows a fast and very sensitive comparison of electrochemically active defect densities of plasma polymer coatings compared to standard corrosion tests that require test duration up to several hundreds of hours. To study the long term stability of the coatings, samples (Cu tracks on polymer — IPC-boards) were stored at 40 °C in 93% r.h. additionally applying 15 V, while the surface resistance was measured at different times within test time of 1000 h. A decrease of the surface resistance is associated with corrosion products forming a short circuit between two Cu tracks.
Fig. 2. Defects visualization on plasma polymer coating on Cu samples by AgNO3 solution.
3. Results and discussion The influence of the plasma pre-treatment on the corrosion protective properties of the plasma polymer coatings is shown in Fig. 2a–c) for identical plasma polymer deposition processes. The images show three samples after the test with AgNO3 solution. For the sample without plasma pre-treatment, a defect density of 90 defects per 25 mm2 was observed (Fig. 2a), while a pre-treatment with an air plasma before coating did not lead to a significant improvement (72 defects per 25 mm2; Fig. 2b). Pre-treatment with a nitrogen plasma containing 3 vol.% of hydrogen results in a significant decrease of the defect density to 9 defects per 25 mm2 (Fig. 2c). For a good protection against corrosive media beside the prevention of defects and migration paths in the coating, the adhesion of the plasma polymer coating on the metallic surface is a key issue. A good adhesion is essential for the prevention of subsurface migration and therefore degradation of the protection properties, especially for long term protection. The adhesion of the plasma polymer coating to the substrate can be qualitatively assessed by close inspection of the precipitated Ag at the defect sites. Coloured rings around the defects appear, when air plasma is used for pre-treatment (not shown). We assume that the coating is detached in these areas from the copper
substrate. The coloured rings are possible results of two different effects: On the one hand a gap shows a phase shift of Δφ = 180°, leading to an effect known as Newton's rings. On the other hand it is possible, that solution with nitrate ions migrates into the gap and causes an oxidation of the copper. This oxidation at the copper surface is accompanied by a colour change. The delamination effect may be due to the weak adhesion of the plasma polymer coating to the air plasma oxidised Cu surface. In case of a pre-treatment with air plasma the circumference of the defect formation due to subsurface migration and delamination is more distinctive than for an untreated or with N2/H2 plasma pre-treated Cu surface. In the last case it is assumed that the increase in chemical stability of the interphase between plasma polymer and Cu oxide-hydroxide (after N2/H2 plasma) results in a better adhesion and therefore in a better protection of the Cu surface. For a more thorough understanding of the surface modification due to the plasma pre-treatment, SEM analyses were performed within 10 min after pre-treatment of the Cu surface. After an air plasma pre-treatment only minor changes in surface topography can be detected directly and after 5 h of the pre-treatment (Fig. 3a and b).
C. Regula et al. / Surface & Coatings Technology 205 (2011) S355–S358
S357
Table. 1 XPS data of PVD copper surface before and after plasma pre-treatment.
Copper reference 20 min after N2/H2-plasma Copper reference 20 min after air-Plasma
Cu(2p3) [at.%]
O (1s) [at.%]
C (1s) [at.%]
N (1s) [at.%]
24 20 28 31
41 40 37 34
34 37 35 31
1 3 0 4
Due to the high reactivity of the surface even for short exposure times (approximately 10 min) between N2/H2 plasma pre-treatment and XPS analysis, a cleaning effect of the reductive process could not be detected by XPS. The long term protection ability of the coating is assessed by the surface resistance of pre-treated and coated copper tracks (IPCboard). While the surface resistance of the reference decreases significantly, the protective paint and the plasmapolymer coating decreases only slightly, shown in Fig. 5.
Fig. 3. Cu surface (a) before an air plasma pre-treatment process. b) One hour after an air plasma pre-treatment process.
Corresponding XPS-analysis (Table 1) showed that the air plasma pre-treatment lead to a minor removal of organic contaminations in contrast to other literature results at low pressure [23,24] or atmospheric pressure plasmas with different plasma sources [25,26]. This might be correlated to the adsorption of carbon containing species during transfer to the XPS-chamber. Deviant from the expected improvement for the corrosion resistance owing to the passivative character (expected increase in oxide layer thickness) of an oxidative plasma pre-treatment, the protective performance of the subsequent deposited coating was not improved. A possible explanation may be the low mechanical stability of the generated Cu oxide [27,28]. More likely an air plasma pre-treatment creates no reactive Cu oxide/hydroxide surface (Fig. 3a and b). A passivative impact of an increased oxide-layer (expected due to air plasma pre-treatment) on the defect density was not detected. Therefore we assume that the adhesion is not improved by this pre-treatment. The SEM pictures (Fig. 4a and b) display a significant change of the surface topology before and after the N2/H2 plasma pre-treatment within one hour under ambient conditions. The observed surface structures in Fig. 4b and c are likely to be carbon containing oxide/ hydroxide species, resulting from an increased reactivity of the surface and the exposure to ambient conditions. Similar surface structures have been reported in [29] after about two years of storage in ambient air or even sea side exposures containing vapor and minerals as carbonate species. Due to the N2/H2 plasma pre-treatment the reactivity of the copper surface is increased, which led to significant changes in surface topology after storage under ambient conditions (Fig. 4c). SEM results were assisted by XPS analysis, which showed an increase of the carbon and oxygen content with storage time (Table 1). Therefore a short interaction time between the plasma pre-treatment and the plasma polymer coating is essential for processes under typical industrial atmospheric conditions. A wider time span will increase the defect density linearly as further experiments showed.
Fig. 4. Cu surface (a) before a reductive pre-treatment process. b) One hour after a reductive pre-treatment process. c) 20 h after a reductive pre-treatment process.
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C. Regula et al. / Surface & Coatings Technology 205 (2011) S355–S358
technical support concerning the APP jet system. Furthermore the financial support of German Federal Ministry of Education and Research (BMBF, FKZ 13N9244) is gratefully acknowledged.
References
Fig. 5. Graph of the surface resistance for coated and uncoated copper tracks under hot wet climate.
4. Conclusion The coating performance of a plasma polymer coating on technical Cu surfaces under hot wet conditions (usually described as corrosion protection for electronic devices) and corrosive environments has been significantly improved by the use of a plasma pre-treatment with a gas mixture of hydrogen and nitrogen. With this pre-treatment the surface resistance decays only slightly and a lower defect density with reduced corrosion creep is observed. This is explained by a more reactive oxide-hydroxide due to N2/H2 plasma pre-treatment leading to an improved and more stable interface between the plasma polymer and the copper surface. The protection performance of a plasma polymer coating is similar or slightly better compared to nowadays paint based protection concepts. Acknowledgement The authors thank F. Eder (Siemens) for the surface resistance measurements, C. Tornow for XPS measurements and J. Degenhardt for
[1] M. Suppa, in: Elekronische Baugruppen Aufbau- und Fertigungstechnik, S. 287–295, Vorträge des DVS/GMM-Tagung, Fellbach. VDE Verlag, 2004. [2] H. Yasuda, B.H. Chun, D.L. Cho, T.J. Lin, D.J. Yang, J.A. Antonelli, Corrosion 52 (1996) 169. [3] S. Sathiyanarayanan, S. Muthukiskan, G. Vankatachar, D.C. Trivedi, Prog org Coat 53 (4) (2005) 297. [4] C.P. Wong, Adv. Polym. Sci. 84 (1988) 63. [5] A. Nazeri, P.P. Trzaskoma, D. Bauer, J. Sol Gel Sci. Technol. 10 (1997) 317. [6] H. Ardelean, I. Frateur, P. Marcus, Corros. Sci. 50 (2008) 1907. [7] H. Koinuma, H. Ohkubo, T. Hashimoto, K. Inomata, T. Shiraishi, A. Miyanaga, S. Hayashi, Appl. Phys. Lett. 60 (1992) 816. [8] A. Schütze, J.Y. Jeong, S.E. Babayan, J. Park, G.S. Selwyn, R.F. Hicks, IEEE Trans. Plasma Sci. 26 (1998) 1685. [9] M. Laroussi, T. Akan, Plasma Process. Polym. 4 (2007) 777. [10] J. Laimer, H. Störi, Plasma Process. Polym. 4 (2007) 266. [11] K. Inomata, H. Ha, K.A. Chaudhary, H. Koinuma, Appl. Phys. Lett. 64 (1994) 46. [12] S.E. Babayan, J.Y. Jeong, V.J. Tu, J. Park, G.S. Selwyn, R.F. Hicks, Plasma Sources Sci. Technol. 7 (1998) 286. [13] T.P. Kasih, S. Kuroda, H. Kubota, Chem. Vap. Depos. 13 (2007) 169. [14] J. Benedikt, V. Raballand, A. Yanguas-Gil, K. Focke, A. von Keudell, Plasma Phys. Control. Fusion 49 (2007) B419. [15] M.H. Han, J.H. Noh, T.J. Lee, J.H. Choi, K.W. Park, H.S. Hwang, K.M. Song, H.K. Baik, Plasma Process. Polym. 5 (2008) 861. [16] V. Hopfe, D.W. Sheel, Plasma Process. Polym. 4 (2007) 253. [17] X. Zhu, F. Arefi-Khonsari, C. Petit-Etienne, M. Tatoulian, Plasma Process. Polym. 2 (2005) 407. [18] A. Churpita, Z. Hubicka, M. Cada, D. Chvostova, L. Soukup, L. Jastrabik, P. Ptacek, Surf. Coat. Technol. 174–175 (2003) 1059. [19] J. Schäfer, R. Foest, A. Quade, A. Ohl, K.D. Weltmann, J. Phys. D Appl. Phys 41 (2008) 194010 S1. [20] M. Noeske, J. Degenhardt, S. Strudthoff, U. Lommatzsch, Int. J. Adhes. Adhes. 24 (2004) 171. [21] U. Lommatzsch, D. Pasedag, A. Baalmann, G. Ellinghorst, H.-E. Wagner, Plasma Process. Polym. 4 (2007) 1041. [22] U. Lommatzsch, J. Ihde, Plasma Process. Polym. 6 (2009) 642. [23] M.A. Baker, Thin Solid films 69 (1) (1980) 359. [24] B. Kegel, H. Schmid, Surf. Coat. Technol. 112 (1–3) (1999) 63. [25] M.C. Kim, S.H. Yang, J.H. Boo, J.G. Han, Surf. Coat. Technol. 174–175 (2003) 839. [26] C.H. Yi, C.H. Jeong, Y.H. Lee, Y.W. Ko, G.Y. Yeom, Surf. Coat. Technol. 177–178 (2004) 711. [27] E. Apen, B.R. Rogers, A. James, J. Sellers, Vac. Sci. Technol. A 16 (3) (1998) 1227. [28] H.K. Yun, K. Cho, J.H. An, C.E. Park, J. Mater. Sci. 27 (1992) 5811. [29] K.P. Fitzgerald, J. Nairn, G. Skennerton, A. Atrens, Corros. Sci. 48 (9) (2006) 2480.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Corrosion protection of copper surfaces by an atmospheric pressure plasma jet treatment Christoph Regula ⁎, Joerg Ihde, Uwe Lommatzsch, Ralph Wilken Fraunhofer Institute for Manufacturing Technology and Advanced Materials, Wiener Str. 12, 28359 Bremen, Germany
a r t i c l e
i n f o
Available online 3 April 2011 Keywords: Atmospheric pressure plasma Copper Corrosion protection Silicon-organic plasma polymer coating
a b s t r a c t The pre-treatment of copper by an atmospheric pressure plasma jet system with different gas mixtures was studied. Their influence on the protection properties of plasma polymer coatings against corrosive environments were examined via an electrochemical test for the visualization of defects in the plasma polymer coating. The defect density could be significantly reduced by the pre-treatment with nitrogenhydrogen plasma. Time between pre-treatment and coating process is of important influence on the defect density as our investigation shows. The coating properties were compared with a typical protective paint system used today for electric boards. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Electronic devices consist of metallic conducting paths and contacts, mostly made from copper (Cu) due to its low electric resistance. These metallic parts have to be protected against deterioration by humidity or corrosive environments, therefore protective paints, e.g. parylene or polyaniline based coatings with thicknesses up to 100 μm are used [1–3]. Disadvantages of these processes are the consumption of solvents, the time necessary for curing and the required amount of energy. Due to the high thickness, protective paints are detrimental for the heat dissipation of the electronic components. Besides a lacquer coating a complete cover consisting of potting or mold compounds is used [4]. In some cases metallic surfaces can be protected by deposition of different metallicoxide layers e.g. [5,6]. Therefore we studied the use of an atmospheric pressure plasma (APP) jet system, operating in the non-equilibrium regime, as an alternative process for the deposition of protective coatings on technical Cu surfaces covered by oxides. The protective properties of the plasma polymer coating on Cu surfaces were studied by the analysis of inhomogeneities and electrochemically active defects in the coating. The generation of thin plasma polymer coatings by our APP jet is an environmentally friendly process, which does not rely on rare gases. The use of such APP jet systems for coating processes is an active research field [7–21]. Former investigations for corrosion protection of aluminium alloys with the same plasma device have been reported in [22].
For the experiments a modified version of an APP jet system (Plasmatreat GmbH, Steinhagen) was used (Fig. 1). This system operates with an excitation frequency between 17 and 25 kHz and a working gas flow rate of 1740 Nl·h− 1. Pre-treatment and coating is performed by moving the nozzle relative to the substrate at a speed between 10 and 20 m/min, which result in typical interaction times in the millisecond range. The samples were pre-treated applying compressed air (CA) or gas mixtures of nitrogen with hydrogen (N2/H2). Subsequently the samples were coated via plasma polymerisation. For all process steps the distance between the nozzle and the substrate was kept constant. For the deposition of the plasma polymer coatings, hexamethyldisiloxane (HMDSO, Wacker, purity N99%) was evaporated and injected into the plasma at the end of the nozzle by using nitrogen as process gas with a flow rate of 60 to 180 Nl·h− 1. The flow rate of the HMDSO was varied between 10 and 50 g/h. Flow rates of precursor and gases were controlled by mass flow controllers (Liqui-Flow (L23V02) and El-Flow (F201CV), Bronkhorst). The typical coating thickness was approximately 450 nm and showed a composition of ~ 25 at.% C, ~26 at.% Si and ~ 49 at.% O, by XPS-measurements. Scanning electron microscopy (SEM, Leo 1530 with a Gemini column, Zeiss) and X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos) of the surface were carried out to detect changes in surface topography and chemical composition. For corrosion tests, samples with 600 nm thick PVD Cu films on silicon wafers b100N (Si-Mat) were used. To study the protective properties of the plasma polymer coating against corrosive media a silver nitrate (AgNO3) solution of defined concentration and volume was applied. The silver ions undergo redox reaction with the Cu, e.g. at defects or conductive paths in the coating, leading to the deposition of silver (Ag) particles.
⁎ Corresponding author. Tel.: + 49 421 2246 682; fax: + 49 421 2246 666. E-mail address: [email protected] (C. Regula). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.03.126
S356
C. Regula et al. / Surface & Coatings Technology 205 (2011) S355–S358
Fig. 1. Scheme of the APP nozzle system for pre-treatment and plasma polymer deposition.
The localised depositions of Ag at the defects were detected by light microscopy and have been quantified by an optical automated analysis (Analysis, Olympus). Test duration of one hour is sufficient due to the high electro-chemical dynamic. After that time the particles continue to grow in size but no additional particles will be generated. This test method allows a fast and very sensitive comparison of electrochemically active defect densities of plasma polymer coatings compared to standard corrosion tests that require test duration up to several hundreds of hours. To study the long term stability of the coatings, samples (Cu tracks on polymer — IPC-boards) were stored at 40 °C in 93% r.h. additionally applying 15 V, while the surface resistance was measured at different times within test time of 1000 h. A decrease of the surface resistance is associated with corrosion products forming a short circuit between two Cu tracks.
Fig. 2. Defects visualization on plasma polymer coating on Cu samples by AgNO3 solution.
3. Results and discussion The influence of the plasma pre-treatment on the corrosion protective properties of the plasma polymer coatings is shown in Fig. 2a–c) for identical plasma polymer deposition processes. The images show three samples after the test with AgNO3 solution. For the sample without plasma pre-treatment, a defect density of 90 defects per 25 mm2 was observed (Fig. 2a), while a pre-treatment with an air plasma before coating did not lead to a significant improvement (72 defects per 25 mm2; Fig. 2b). Pre-treatment with a nitrogen plasma containing 3 vol.% of hydrogen results in a significant decrease of the defect density to 9 defects per 25 mm2 (Fig. 2c). For a good protection against corrosive media beside the prevention of defects and migration paths in the coating, the adhesion of the plasma polymer coating on the metallic surface is a key issue. A good adhesion is essential for the prevention of subsurface migration and therefore degradation of the protection properties, especially for long term protection. The adhesion of the plasma polymer coating to the substrate can be qualitatively assessed by close inspection of the precipitated Ag at the defect sites. Coloured rings around the defects appear, when air plasma is used for pre-treatment (not shown). We assume that the coating is detached in these areas from the copper
substrate. The coloured rings are possible results of two different effects: On the one hand a gap shows a phase shift of Δφ = 180°, leading to an effect known as Newton's rings. On the other hand it is possible, that solution with nitrate ions migrates into the gap and causes an oxidation of the copper. This oxidation at the copper surface is accompanied by a colour change. The delamination effect may be due to the weak adhesion of the plasma polymer coating to the air plasma oxidised Cu surface. In case of a pre-treatment with air plasma the circumference of the defect formation due to subsurface migration and delamination is more distinctive than for an untreated or with N2/H2 plasma pre-treated Cu surface. In the last case it is assumed that the increase in chemical stability of the interphase between plasma polymer and Cu oxide-hydroxide (after N2/H2 plasma) results in a better adhesion and therefore in a better protection of the Cu surface. For a more thorough understanding of the surface modification due to the plasma pre-treatment, SEM analyses were performed within 10 min after pre-treatment of the Cu surface. After an air plasma pre-treatment only minor changes in surface topography can be detected directly and after 5 h of the pre-treatment (Fig. 3a and b).
C. Regula et al. / Surface & Coatings Technology 205 (2011) S355–S358
S357
Table. 1 XPS data of PVD copper surface before and after plasma pre-treatment.
Copper reference 20 min after N2/H2-plasma Copper reference 20 min after air-Plasma
Cu(2p3) [at.%]
O (1s) [at.%]
C (1s) [at.%]
N (1s) [at.%]
24 20 28 31
41 40 37 34
34 37 35 31
1 3 0 4
Due to the high reactivity of the surface even for short exposure times (approximately 10 min) between N2/H2 plasma pre-treatment and XPS analysis, a cleaning effect of the reductive process could not be detected by XPS. The long term protection ability of the coating is assessed by the surface resistance of pre-treated and coated copper tracks (IPCboard). While the surface resistance of the reference decreases significantly, the protective paint and the plasmapolymer coating decreases only slightly, shown in Fig. 5.
Fig. 3. Cu surface (a) before an air plasma pre-treatment process. b) One hour after an air plasma pre-treatment process.
Corresponding XPS-analysis (Table 1) showed that the air plasma pre-treatment lead to a minor removal of organic contaminations in contrast to other literature results at low pressure [23,24] or atmospheric pressure plasmas with different plasma sources [25,26]. This might be correlated to the adsorption of carbon containing species during transfer to the XPS-chamber. Deviant from the expected improvement for the corrosion resistance owing to the passivative character (expected increase in oxide layer thickness) of an oxidative plasma pre-treatment, the protective performance of the subsequent deposited coating was not improved. A possible explanation may be the low mechanical stability of the generated Cu oxide [27,28]. More likely an air plasma pre-treatment creates no reactive Cu oxide/hydroxide surface (Fig. 3a and b). A passivative impact of an increased oxide-layer (expected due to air plasma pre-treatment) on the defect density was not detected. Therefore we assume that the adhesion is not improved by this pre-treatment. The SEM pictures (Fig. 4a and b) display a significant change of the surface topology before and after the N2/H2 plasma pre-treatment within one hour under ambient conditions. The observed surface structures in Fig. 4b and c are likely to be carbon containing oxide/ hydroxide species, resulting from an increased reactivity of the surface and the exposure to ambient conditions. Similar surface structures have been reported in [29] after about two years of storage in ambient air or even sea side exposures containing vapor and minerals as carbonate species. Due to the N2/H2 plasma pre-treatment the reactivity of the copper surface is increased, which led to significant changes in surface topology after storage under ambient conditions (Fig. 4c). SEM results were assisted by XPS analysis, which showed an increase of the carbon and oxygen content with storage time (Table 1). Therefore a short interaction time between the plasma pre-treatment and the plasma polymer coating is essential for processes under typical industrial atmospheric conditions. A wider time span will increase the defect density linearly as further experiments showed.
Fig. 4. Cu surface (a) before a reductive pre-treatment process. b) One hour after a reductive pre-treatment process. c) 20 h after a reductive pre-treatment process.
S358
C. Regula et al. / Surface & Coatings Technology 205 (2011) S355–S358
technical support concerning the APP jet system. Furthermore the financial support of German Federal Ministry of Education and Research (BMBF, FKZ 13N9244) is gratefully acknowledged.
References
Fig. 5. Graph of the surface resistance for coated and uncoated copper tracks under hot wet climate.
4. Conclusion The coating performance of a plasma polymer coating on technical Cu surfaces under hot wet conditions (usually described as corrosion protection for electronic devices) and corrosive environments has been significantly improved by the use of a plasma pre-treatment with a gas mixture of hydrogen and nitrogen. With this pre-treatment the surface resistance decays only slightly and a lower defect density with reduced corrosion creep is observed. This is explained by a more reactive oxide-hydroxide due to N2/H2 plasma pre-treatment leading to an improved and more stable interface between the plasma polymer and the copper surface. The protection performance of a plasma polymer coating is similar or slightly better compared to nowadays paint based protection concepts. Acknowledgement The authors thank F. Eder (Siemens) for the surface resistance measurements, C. Tornow for XPS measurements and J. Degenhardt for
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